Tuning circuit for radio transmitters



March 23,

J. M. DODDS ET AL TUNING CIRCUIT FOR RADIO TRANSMITTERS Filed Dec. 29, 1945 Patented Mar. 23, 1948 TUNING CIRCUIT FOR RADIO TRANSMITTERS John Matheson Dodds, Timperley, and John Heywood Ludlow, Bowdon, England Application December 29, 1943, Serial No. 516,110 `In Great Britain July `10, 1940 2 Claims. (Cl. Z50-40) This .invention relates to radio transmitters kof high power/wavelength ratio, that is, power 'of the order of at least ten kilowatts and wavelengths of materially less than, say, thirty metres, and specifically concerns the condensers of 'very short wave tuning circuits in which are included the electrodes of the high power valve or vaives, more particularly, but not exclusively, of the screened-grid type in push-pull or quasi pushpull arrangements which have been successfully used in the manner described, inter alia, in the specification of British Letters Patent No. 476,188, and in the paper by Burch and Sykes, given in Proc. `I. E. vol. 7'?, No. 463, .July i935, pages 1294140.

The present invention is -.based on the follow ing considerations, viz:

When two electrodes of a power valve are in eluded in a resonant circuit operating at very short wavelength having regard to the power, the electrostatic capacity between said electrodes per se constitutes the major portion of the total capacity in the resonant tuning circuit in which the large high frequency current lflows. This invention is lessentially concerned with the provision of an additional variable capacity for bringing said circuit vexactly into the desired resonance without greatly increasing the minimum value of the wavelength to which the circuit may be required to be tuned, and in partcula.; without forming, or tending to form, subsidiary circuits which may inoonveniently give rise to spurious resonances. it is to be understood that the invention is equally applicable to a resonant circuit of a high power-wavelength ratio transmitter, which circuit does not include a valve but an evacuated condenser of iixcd capacity, or which circuit, as in the disclosures `of the aforesaid prior art, includes a single valve and .an evacuated fixed capacity condenser in balanced or quasi push-pull arrangement. It will be appreciated 'by those skilled in the art that on the one hand it is in the present state of knowledge so diilicult to construct an evacuated condenser of variable capacity that it is desirable to use one or xed capacity and employ in conjunction with it a variable capacity condenser of the mov ing plate air dielectric type, whilst on the other hand it is desirable to employ an `evacuated condenser on account of the fact that it can be compactly constructed to withstand high voltages, to have small losses and to have low impedance leads to its electrodes, all as is desirable in transmitters of high power/wavelength ratio. The present invention is limited to the Ause of an evacuated .fixed Ycondenser in the aforesaid cases, that is, where either there is no valve or there is one valve as well as an evacuated fixed condenser. Furthermore such a xed capacity .evacuated condenser is the equivalent of the valve in .the respects that its plates .are conveniently coaxially cylindrical, whilst at least one of its plates ,must have a support comparable, for example, with that ofthe anode or a grid of the valve.

The invention is inherently applicable, as will Abe vper :se common knowledge to those skilled in the art, t0 a transmitter circuit wherein lthe additional variable capacity is used either for wavelength changing, alone 01 in conjunction with substitution ol variation of inductance coils and/or iixed condensers for the sole or the final adjustment .(or trimming), respectively, of the circuit exactly into .the desired resonance. That is to say the variable condenser in question is not of the ,pi'eset"y type subject to adjustment once for all, but essentially has an operating mechanism l2 actuable `or controllable at any time from the exterior of the enclosure of the transmitter.

It will be understood, moreover, that at the very low wavelengths and also the high voltages here .in question, the variable condenser has plates oi considerable mutual area generally of the order of square feet rather than of the order of square centmetros or square inches, and that a considerable variation in its capacity is required in relation to the voltage it has to withstand.

The present invention resides in part in the selection of a condenser of the variable gap type by which is herein to be understood to mean a condenser vwhose capacity is changed at least mainly oy varying the distance between its plates (of which there are preferably only two), as distinct from a condenser whose ca pacity is changed 'by varying the area of mutual overlap between its plates the gap between which is substantially constant. Therefore variable condensers of the cylindrical type are excluded from the scope of the present invention; however the plates or the variable condenser need not be completely planar, although preferably they are so.

It is pointed out that Yat the very low wavelengths in question the inductance of the resonant circuit is partly, 'and sometimes mainly, but always materially, constituted by the unavoidable inductance of the high frequency current conducting lead or leads from the electrode or electrodes proper to the termin-als of the one or more valves or evacuated fixed condensers and also by the external connecting conductor or conductors which may be a part or parts of screening boxes: for the minimum wavelengths of the transmitter circuit the inductance coil proper may be reduced to a straight bar between the aforesaid terminals. It has also to be pointed out that the high frequency current carrying inductance of the members just mentioned provides a partial or complete loop which has an axis whose position, and particularly its direction, are all defined, and it is this partial or complete loop which is hereinafter referred to simply as inductance loop.

An important object of the present invention is to reduce the minimum wavelength to which the circuit may be tuned having regard to the large high frequency power therein.

The resonant tuning circuit according to the invention has or comprises the following features in combination with the one or more valves or evacuated fixed condensers and the added inductance coil member:

(a) The or each variable condenser of the variable gap type has its plates in parallel planes which are parallel with the axis of the inductance loop,

(b) The fixed plate of the or each condenser (a) is attached to cr'even forms part of that termina] of the valve or fixed condenser which is at very high radio frequency potential,

(c) The moving plate of the or each condenser (a) is of similar area to the fixed plate, and is provided with mechanism whereby it may be caused to move by external actuation or control, and

(d) Said moving plate (c) is electrically connected to a fixed terminal of the circuit by at least one movable or flexible connection having considerable eiiective length in a direction parallel with the axis of said inductance loop whereby low impedance is offered to the high frequency charging current flowing to the moving plate. Said low impedance electrical connection is preferably such that, at least when the variable condenser is in its position of maximum capacity, said connection shall be close to an electrode or electrode support with which the moving plate is or may be associated so that the inductance loop ineluded between said connection and said electrode or support is a minimum.

The main application of the invention, is, as hereinbefore indicated, for the tuning of the anode or the grid circuit of a valve amplier, and more particularly the anode circuit thereof. These circuits must, at the very short wavelengths in question, for example from five to iifteen metres, be screened from one another by a screening plate and are in fact in practical arrangements enclosed in respective screening boxes which may have one common wall plate or partition. In carrying out the invention in such application the low impedance electrical connection between the moving condenser plate may be made to said common wall plate in such a way that in the minimum capacity position of the moving plate it lies adjacent a screening box wall plate which is at right angles to said common wall plate. In this arrangement the moving condenser plate conveniently comprises two portions, namely a main portion, which is the larger and may be said generally to co-operate with the xed plate of the condenser, Whilst the other plate is hinged to one edge of the main plate by a long low impedance connection and has its i other edge hinged by another long low impedance connection to the common wall plate.

In any form the long low impedance connection is conveniently a hinge member, or a plurality of hinges, which is or are short-circuited by continuous liexible conducting means such as longitudinally corrugated copper strip I3 soldered to the plates on each side of each hinge.

A further advantage of the condenser arrangement according to the invention is that, when used in a transmitter which is required to operate over a wide range of wavelengths, when resonance is obtained with the condenser near its maximum value for the longer wavelengths and near its minimum value for the shorter wavelengths, the case of adjustment to a given resonant frequency is greatly improved. The main advantage of the invention, however, is as above indicated, namely that the condenser arrangement renders the formation, or tendency to the formation, of secondary circuits with associated spurious resonances unlikely, whereas the forms of connection heretofore known between the moving condenser plate form or tend to form such circuits and/or have such high impedance that the effectiveness of the condenser is impaired, usually with a loss in the amount otherwise possible of the power output.

Furthermore in carrying out the invention it is preferable to dispose the variable condenser on the side of the valve opposite to that where the tuning inductance member is located, in a manner per se known. Thus in the case of a pushpull or quasi push-pull circuit the valves or the valve and fixed condenser may be arranged with their axes parallel and have their anodes connected together by the inductance member, which at the lower wavelengths may be a straight wide strap extending directly between the anodes or terminals, the variable kcondensers being arranged on the remote sides of the valves. It is to be appreciated, however, that the condensers may be arranged between the valves, but this may in many cases be less advantageous owing to the presence between them of the inductance member and particularly when switchgear is provided for wave-changing by substituting different inductance members or equivalently. Y

The various links and shafts which may be necessary to provide the required parallel movement of the main plates of both condensers from a single control are preferably mounted close to either the condenser plates themselves or to the metallic plates forming the screening enclosure.

Figure 1 in the drawing is a top plan view of the apparatus as shown in Figure 2 but with the moving plates of the condensers in outward or low capacity position,

Figure 2 is a side elevation of the apparatus, and

Figure 3 is an enlarged axial sectional view of the left-hand valve anode and support of Figure 2 showing the cylindrical screen grid and its cylindrical support.

In the accompanying drawing representing 'in side elevation apparatus in accordance with the invention Al and A2 represent the anodes of a pair of valves in push-pull, or quasi push-pull, relation, these anodes being connected by tubular insulators Bl and B2 to the screening grid terminals Tl and T2 which are electrically bound to the partition S of the anode circuit screening box, having sides represented by Si, and the screening box having sides S2 of the grid circuit of the valves, which circuit is not illustrated.

The valves may be in accordance with British Letters Patent No. 414,753.

At C is shown a horizontal strap connection which is the inductance member completing the anode circuit. The position of the axis of the anode inductance loop is represented approximately by the dot X in Figure 2 and the line X-X in Figure l. It may be mentioned that if the valves are of the screened grid type such as in accordance with the British Letters Patent just above mentioned, the capacity between anode and screen grid, which is the xed capacity in question, is high so that the added inductance member may comprise the straight strap C for the lower wavelengths. However, the member C may be replaced by a member having helical turns on an axis at right angles to the axes of the two valvesl but such helix axis is not the one involved in the present invention which axis is perpendicular to the paper in Figure 2 and parallel to the plane of the paper in Figure 1 and is the axis of the inductance loop or part loop constituted essentially .by the electrode supports extending within the insulators BI and B2 from the terminals Tl and T2 to the grid electrodes within the anodes Al and A2, and by the intervening part of the partition S, this loop (or part loop) being essentially formed so that it offers a minimum impedance to the heavy high frequency currents flowing in the circuit which includes the interelectrode capacity of the valves.

At Pl and P2 are shown the fixed plates of the tuning condenser, these being secured with good conductance to the anodes AI and A2 as shown. At MI and M2 are shown the main moving plates of the condensers. These plates along their lower edges are hinged at Hl and H2 to the auxiliary plates FI and F2, which in turn are hinged at H3 and H4 upon the surface of the screen plate S close to the valve terminals Tl and T2. In the full line position of the condenser plates the condenser capacity is highest Whilst in the dotted line position the condenser capacity is at its minimum. As hereinbefore set forth, all the condenser plates are parallel with the inductance loop X and said plates and also all the high conductance hinges thereof have considerable length in the direction parallel with said axis. In order that the impedance of the partial inductance loop in question shall be low the partition S is of copper or other good conductivity metal as are also the condenser plates.

It will be appreciated that instead of two similar valves in push-pull as above described with reference to the accompanying drawing, there maybe a single valve and a symmetrically arranged condenser as described in British Patent Speciiication No. 476,188 aforesaid and shown more particularly in Figs. 1, 2 and 3 thereof, and also in the paper by Burch and Sykes aforesaid.

We claim:

1. In a radio transmitter for ultra high frequencies; a valve including an anode, a grid terminal spaced from said anode, a `screen grid associated with said anode and a metallic support for said screen grid electrically associating said screen grid with said grid terminal adapted to provide an anode inductance path of low inductance; a

condenser having a xed plate connected to the anode, a cooperating main movable plate, and means to move the main movable plate through a range of positions while maintaining it parallel to the iixed plate; said moving means including an auxiliary plate hingedly and electrically connected along one edge to the main movable plate and along its opposite edge to said grid terminal so as to electrically associate the main movable plate and the auxiliary plate with said grid terminal; said plates being so dimensioned as to maintain the impedance of the anode inductance loop at a low value.

2. In a radio transmitter for ultra high frequencies; a pair of valves arranged side by side; each of said valves having an anode inductance path including a valve anode, a grid terminal spaced from said anode, a screen grid associated with said anode and a metallic support for said screen grid electrically associating said screen grid with said grid terminal; an inductance member extending .between said valve anodes; a conductive means extending between said valve grid terminals; an anode inductance loop comprising said anode inductance paths, said inductance member between said valve anodes and said conductive means between said valve grid terminals; adjustable condensers positioned for electrical coupling with said anode inductance loop and arranged electrically in series and adapted to vary the tuning of said ano-de inductance loop; each condenser including a fixed plate connected to the anode of the associated valve, a movable cooperating main plate and means to move the movable plate through a wide range of positions while maintaining it parallel to the xed plate; said moving means including an auxiliary plate hingedly connected along one edge to the main movable plate and along the opposite edge to one of said inductance members forming said anode inductance loop so as to electrically associate the main movable plate and the auxiliary plate with the anode inductance loop; said plates being so dimensioned as to maintain the impedance of the anode inductance loop at a low value.

JOHN MATHIESON DODDS. JOHN HEYWOOD LUDLOW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,740,131 Werner Dec. 17, 1929 1,960,058 Lynn May 22, 1934 2,085,838 Usselman July 6, 1937 2,120,518 Dreger, Jr. June 14, 1938 2,277,638 George Mar. 24, 1942 FOREIGN PATENTS Number Country Date 564,836 France Oct. 30, 1923 OTHER REFERENCES QST., Firing Up on the Newly-Opened Ultra- High Frequencies, by Ross A. Hull, September 1934, pp. 13, 14, 15. 250-40 Lit. 

